I also would need the manual Reader mode. I tried fiddling around with the Chrome flags, but this is also not a solution. Even when forced, sometimes the button is there, sometimes it's not. Sometimes the reader dialog is shown, sometimes it's not. Even for the same website. So it is totally unreliable.
Are there any plans to add an option to have this button always available so that reader mode can be triggered manuelly by the user like it is available for e. g. Edge? It's the only drawback so far that I have with Vivaldi unfortuinately...